type-II Materials Research Articles

Enhanced Ionic Power Generation via Light-Driven Active Ion Transport Across 2D Semiconductor Heterostructures.

2D semiconductor heterostructures exhibit broad application prospects. However, regular nanochannels of heterostructures rarely caught the researcher's attention. Herein, a metal-organic framework (i.e., Cu3(HHTP)2) and transition metal dichalcogenides (i.e., MoS2)-based multilayer van der Waals heterostructure (i.e., Cu3(HHTP)2/MoS2) realized band alignment-dominated light-driven ion transport and further light-enhanced ionic energy generation. High-density channels of the heterostructure provide high-speed pathways for ion transmembrane transport. Upon light illumination, a net ionic flow occurs at a symmetric concentration, suggesting a directional cationic transport from Cu3(HHTP)2 to MoS2. This is because Cu3(HHTP)2/MoS2 heterostructures containing type-II band alignment can generate photovoltaic motive force through light-induced efficient charge separation to drive ion transport. After introducing into the ionic power generation system, the maximum power density under illumination can achieve notable improvement under different concentration differences. In addition to the photovoltaic motive force, type-II band alignment and material defect capture-induced surface charge increase also raise ion selectivity and flux, greatly facilitating ionic energy generation. This work demonstrates that 2D semiconductor heterostructures with rational band alignment can not only be a potential platform for optimizing light-enhanced ionic energy harvesting but also provide a new thought for biomimetic iontronic devices.

Two-dimensional antiferromagnetic type-II Weyl fermions in monolayer ZnSb and type-I Weyl fermions and topological insulator in bilayer ZnSb

Two-dimensional (2D) topological materials have attracted extensive attention due to the discovery of graphene. In addition, their counterparts in magnetic materials are also a fascinating subject of research. However, it remains challenging to find 2D materials that have the desired topological phases. Here, based on first-principles calculations, we investigate the properties of a newly synthesized 2D material, monolayer/bilayer (ML/BL) ZnSb. We find that the ML ZnSb is antiferromagnetic (AFM) and that its magnetism can be changed by an external strain, from AFM to ferromagnetic. Without any external strain, the ML ZnSb can be considered as an almost ideal 2D type-II Weyl material with a pair of type-II Weyl fermions exactly located at the Fermi level. When taking into account spin–orbit coupling (SOC), a gap can be introduced at the Weyl point. We further show that by applying a biaxial strain to the BL ZnSb material, it is possible to open different gaps at the Weyl points, leading to a topological phase transition without breaking symmetry. When SOC is included, these Weyl points become topological insulators with a large bandgap, where the helical edge states can be identified. Overall, this study provides an example of a material that can be used to explore the physical consequences of 2D topological phases and provide a promising candidate for future research in this field.

Orbitally driven spin reorientation in Mn-doped YBaCuFeO5

The oxygen-deficient layered perovskite YBaCuFeO5 (YBCFO) is a rare type-II multiferroic material where ferroelectricity, driven by incommensurate spiral magnetic order, is believed to be achievable up to temperatures higher than room temperature. A cycloidal spiral order rather than a helical spiral order is an essential ingredient for the existence of ferroelectricity in this material. Motivated by a recent experimental study on Mn-doped YBCFO where the spiral plane was observed to cant more towards the crystallographic c-axis upon Mn doping at Fe sites than in the parent compound, we performed a detailed theoretical investigation using density functional theory calculations to understand the mechanism behind such spin reorientation. Our total energy calculations, within the generalized gradient approximation plus U plus spin–orbit coupling approach, reveal that Fe/Cu spin moments indeed align more towards the c-axis in Mn-doped YBCFO than in the parent compound. The largest exchange interaction (Cu–Cu in the a−b plane) is observed to decrease systematically with Mn doping concentration, reflecting the lowering of the transition temperature seen in experiments. Further, the inter-bilayer Cu–Mn exchange interaction becomes ferromagnetic in Mn-doped YBCFO, whereas the corresponding Cu–Fe exchange is antiferromagnetic in the parent compound, giving rise to frustration in the commensurate magnetic order. Most importantly, our electronic structure calculations reveal that in Mn-doped YBCFO because of hybridization with the Mn dz2 orbital, the highest occupied Cu orbital becomes the dz2 orbital as opposed to the dx2−y2 orbital in the parent compound. Therefore, we believe that the occupancy of the out-of-plane-oriented Cu dz2 orbital in place of the planar dx2−y2 orbital along with the frustrating exchange interaction drives the spins to align along the c-axis in Mn-doped YBCFO.

Graded Longitudinal Fins Having Spatially Varying Temperature-Dependent Thermophysical Properties

This paper reports transient responses of graded longitudinal fins subject to step change in base temperature and base heat flux wherein the graded fin materials are theorized to have spatial- and temperature-dependent thermal conductivity. Microstructure variations in graded materials (GMs) are addressed by axially varying the thermal conductivity; because GMs are potentially high-temperature application materials, consequently, thermal conductivity and heat generation are, respectively, assumed as polynomial and linear functions of temperature. Additionally, most of the applicable pragmatic fluid regimes are accounted for using the power law convection coefficient. The numerical solution of a typical nonlinear governing differential equation is obtained by using a particle tracking-based method called the lattice Boltzmann method (LBM). The LBM is a mesoscopic-based simulation method centered around the principles of kinetic theory and statistical mechanics. The LBM formulation accompanied with the in-house MATLAB code of the aforesaid problem with varying parameters is reported; also, it is validated with a previously available solution. The foregoing analysis is carried out to enhance the performance of a fin by using the superior thermomechanical property of graded materials. Furthermore, the inclusion of temperature-dependent thermophysical properties and heat generation will provide more accurate design data. The reported graph reveals that, even though a linear GM fin tip possesses thermal conductivity that is 25% less in magnitude in comparison to the Type-II homogeneous material (HM-2), the GM fin always yields a higher fin tip temperature because of grading. In addition, the tip temperature deficits between GMs and HM-2 proportionally increase from 0.4 to 2.1% for values of increasing from 0.1 to 2.0, respectively, for step changes in temperature; whereas in the case of the step change base flux, the deficits increase from 8.72 to 12.1% for values of decreasing from 3.0 to 1.0, respectively.

Signatures of Lifshitz transition in the optical conductivity of two-dimensional tilted Dirac materials

Lifshitz transition is a kind of topological phase transition in which the Fermi surface is reconstructed. It can occur in the two-dimensional (2D) tilted Dirac materials when the energy bands change between the type-I phase ($0<t<1$) and the type-II phase ($t>1$) through the type-III phase ($t=1$), where different tilts are parametrized by the values of $t$. In order to characterize the Lifshitz transition therein, we theoretically investigate the longitudinal optical conductivities (LOCs) in type-I, type-II, and type-III Dirac materials within linear response theory. In the undoped case, the LOCs are constants either independent of the tilt parameter in both type-I and type-III phases or determined by the tilt parameter in the type-II phase. In the doped case, the LOCs are anisotropic and possess two critical frequencies determined by $\omega=\omega_1(t)$ and $\omega=\omega_2(t)$, which are also confirmed by the joint density of state. The tilt parameter and chemical potential can be extracted from optical experiments by measuring the positions of these two critical boundaries and their separation $\Delta\omega(t)=\omega_2(t)-\omega_1(t)$. With increasing the tilting, the separation becomes larger in the type-I phase whereas smaller in the type-II phase. The LOCs in the regime of large photon energy are exactly the same as that in the undoped case. The type of 2D tilted Dirac bands can be determined by the asymptotic background values, critical boundaries and their separation in the LOCs. These can therefore be taken as signatures of Lifshitz transition therein. The results of this work are expected to be qualitatively valid for a large number of 2D tilted Dirac materials, such as 8-\emph{Pmmn} borophene monolayer, $\alpha$-SnS$_2$, TaCoTe$_2$, TaIrTe$_4$, and $1T^\prime$ transition metal dichalcogenides, due to the underlying intrinsic similarities of 2D tilted Dirac bands.

Bandgap engineering of ZnX (X = O, S, Se, Te) QDs/Graphene nanocomposites: Towards the designing of a highly efficient light-harvesting device

Increasing the power conversion efficiency (PCE) of photovoltaic devices (PVDs) is a topic of fundamental importance not only for academia but also for industry. In this study, we systematically investigated the electronic structure of ZnX (X = O, S, Se, and Te) quantum dots (QDs)/Graphene (G) nanohybrid systems by employing the density functional method. To understand the key issues, associated with photoinduced charge generation, generated charge separation, charge transfer process, recombination probability, and variations of PCE, we have considered different combinations of hybrid nanostructures composed with ZnX QD and G. The tuning of the electronic properties of hybrid systems has been done as a function of the size of ZnX QD's, chalcogenides, and hydrogen coverage percentage on G to design a highly efficient hybrid solar cell. We have also explored the photovoltaic efficiency of ZnX/ZnX’ (X/X' = O, S, Se, and Te) core/shell QDs/G to investigate the effect of core/shell QDs in composite nanomaterials. Bandgaps of ZnX QDs/G nanocomposites are very low (0.028eV–0.158 eV) which implies the probability of recombination of photo-induced separated charges at the heterojunction region is very high. After passivation with hydrogen atoms partially on the G, this unsavory circumstance is overcome. We have performed the bandgap and band alignment engineering of ZnX QDs/G hybrid nanostructures by varying the size of QD's and H-coverage percentage on G to form a type-II material which is very important to reduce the recombination rate and ultimately increase the PCE of the hybrid solar cells. Considering hydrogenated graphene (HG)-ZnX QDs nanocomposites we were achieving high PCE values (up to 30.25%), which implies our explored materials efficiency is exceptionally good and competitive with recently explored tandem/hybrid/latest perovskite solar cells.

Ideal type-II Weyl points in twisted one-dimensional dielectric photonic crystals.

We proposed an one-dimensional layer-stacked photonic crystal using anisotropic materials to realize ideal type-II Weyl points. The topological transition from Dirac to Weyl points can be clearly observed by tuning the twist angle between layers. Also, on the interface between the photonic type-II Weyl material and air, gapless surface states have been demonstrated in an incomplete bulk bandgap. By breaking parameter symmetry, these ideal type-II Weyl points would transform into the non-ideal ones, exhibiting topological surface states with single group velocity. Our work may provide a new idea for the realization of photonic semimetal phases by utilizing naturally anisotropic materials.

Multiferroicity in a quasi-one-dimensional magnet MnSb2Se4

We report a comprehensive study of the structural, magnetic, and multiferroic properties of a quasi-one-dimensional magnet, MnSb2Se4, crystallizing in a centrosymmetric monoclinic structure. Magnetic and heat capacity measurements revealed that this compound undergoes a long-range antiferromagnetic ordering at TN = 22.5 K. A dielectric anomaly appeared at the TN in the absence of a magnetic field, and, an applied magnetic field of 9 T induced another dielectric anomaly just below the TN. Furthermore, the pyroelectric and DC-bias current measurements confirmed that these anomalies are associated with spontaneous electric polarization, whose magnitude becomes almost double under an applied field of 9 T. These results demonstrated that MnSb2Se4 is a type-II multiferroic material with strong magnetoelectric coupling. Moreover, a frequency-dependent dielectric anomaly observed at 5 K in the real component of the dielectric permittivity and loss data indicates relaxor behavior.

Realization of a transition between type-I and type-II Dirac semimetals in monolayers

The phase transition between type-I and type-II Dirac semimetals will reveal a series of significant physical properties because of their completely distinct electronic, optical, and magnetic properties. However, no mechanism and materials have been proposed to realize the transition to date. Here, we propose that the transition can be realized in two-dimensional (2D) materials consisting of zigzag chains by tuning external strains. The origination of the transition is that some orbital interactions in zigzag chains vary drastically with structural deformation, which changes dispersions of the corresponding bands. Two 2D nanosheets, monolayer phosphorous nitride and AsN, are searched out to confirm the mechanism by using first-principles calculations. They are intrinsic type-I or type-II Dirac materials and transit to another type of Dirac materials by external strains. In addition, a possible routine is proposed to synthesize the new 2D structures.

Single-crystal investigations on the multiferroic materialLiFe(WO4)2

The crystal and magnetic structure of multiferroic LiFe(WO$_4$)$_2$ were investigated by temperature and magnetic-field dependent specific heat, susceptibility and neutron diffraction experiments on single crystals. Considering only the two nearest-neighbour magnetic interactions, the system forms a $J_1$, $J_2$ magnetic chain but more extended interactions are sizeable. Two different magnetic phases exhibiting long-range incommensurate order evolve at $T_{\text{N}1}\approx 22.2 \text{ K}$ and $T_{\text{N}2}\approx 19 \text{ K}$. First, a spin-density wave develops with moments lying in the $ac$ plane. In its multiferroic phase below $T_{\text{N}2}$, LiFe(WO$_4$)$_2$ exhibits a spiral arrangement with an additional spin-component along $b$. Therefore, the inverse Dzyaloshinskii-Moriya mechanism fully explains the multiferroic behavior in this material. A partially unbalanced multiferroic domain distribution was observed even in the absence of an applied electric field. For both phases only a slight temperature dependence of the incommensurability was observed and there is no commensurate phase emerging at low temperature or at finite magnetic fields up to $6\text{ T}$. LiFe(WO$_4$)$_2$ thus exhibits a simple phase diagram with the typical sequence of transitions for a type-II multiferroic material.

Manipulating Weyl quasiparticles by orbital-selective photoexcitation in WTe2

Optical control of structural and electronic properties of Weyl semimetals allows development of switchable and dissipationless topological devices at the ultrafast scale. An unexpected orbital-selective photoexcitation in type-II Weyl material WTe2 is reported under linearly polarized light (LPL), inducing striking transitions among several topologically-distinct phases mediated by effective electron-phonon couplings. The symmetry features of atomic orbitals comprising the Weyl bands result in asymmetric electronic transitions near the Weyl points, and in turn a switchable interlayer shear motion with respect to linear light polarization, when a near-infrared laser pulse is applied. Consequently, not only annihilation of Weyl quasiparticle pairs, but also increasing separation of Weyl points can be achieved, complementing existing experimental observations. In this work, we provide a new perspective on manipulating the Weyl node singularity and coherent control of electron and lattice quantum dynamics simultaneously.

MoTe2 as a natural hyperbolic material across the visible and the ultraviolet region

Hyperbolic materials are of particular interest for the next generation of photonic and optoelectronic devices. Since artificial metamaterials are intrinsically limited by the size of their nanostructured components, there has been a hunt for natural hyperbolic materials in the last few years. In a first-principles work based on density-functional theory and many-body perturbation theory, we investigate the fundamental dielectric response of MoTe2 in monolayer, bilayer, and bulk form, and find that it is a natural type-II hyperbolic material with low losses between 3 and 6 eV. Going from the monolayer to the bulk, the energy window of hyperbolic dispersion is blue-shifted by a few tenths of an eV. We show that excitonic effects and optical anisotropy play a major role in the hyperbolic behavior of MoTe2. Our results confirm the potential of layered materials as hyperbolic media for opto-electronics, photonics, and nano-imaging applications.

Computational screening of vdWs heterostructures of BSe with MoSe2 and WSe2 as sustainable hydrogen production materials

Recently, fabricating type-II vertical van der Waals (vdWs) heterostructure is a promising material for hydrogen production. The absorption capability, charge density distributions, band alignments and electronic properties of the monolayers and heterostructures are systematically investigated using computational studies. Using ab initio molecular dynamics, binding energy and phonon calculations, the stability of the heterostructures are verified. Both heterostructures are type-II materials, which can increase the separation of charge carriers. Moreover, the charge density difference and the potential drop across the interface of MSe2/BSe creates a high built-in electric field that can prevent the recombination of charge carriers. We found that the visible-light optical properties of both heterostructures are much enhanced with suitable bandgap energy for water splitting. The band alignment suggests that the heterostructures straddle water redox potentials in acid solutions (0 < pH < 7). Our study predicted that MSe2/BSe vdW heterostructures have great potential for photocatalytic hydrogen production.

Surface effects in superconductors with corners

We review some recent results on the phenomenon of surface superconductivity in the framework of Ginzburg-Landau theory for extreme type-II materials. In particular, we focus on the response of the superconductor to a strong longitudinal magnetic field in the regime where superconductivity survives only along the boundary of the wire. We derive the energy and density asymptotics for samples with smooth cross section, up to curvature-dependent terms. Furthermore, we discuss the corrections in presence of corners at the boundary of the sample.

Giant pressure-enhancement of multiferroicity in CuBr2

Type-II multiferroic materials, in which ferroelectric polarization is induced by inversion non-symmetric magnetic order, promise new and highly efficient multifunctional applications based on the mutual control of magnetic and electric properties. Although this phenomenon has to date been limited to low temperatures, here we report a giant pressure-dependence of the multiferroic critical temperature in CuBr$_2$. At 4.5 GPa, $T_\mathrm{C}$ is enhanced from 73.5 to 162 K, to our knowledge the highest value yet reported for a non-oxide type-II multiferroic. This growth shows no sign of saturating and the dielectric loss remains small under these high pressures. We establish the structure under pressure and demonstrate a 60\% increase in the two-magnon Raman energy scale up to 3.6 GPa. First-principles structural and magnetic energy calculations provide a quantitative explanation in terms of dramatically pressure-enhanced interactions between CuBr$_2$ chains. These large, pressure-tuned magnetic interactions motivate structural control in cuprous halides as a route to applied high-temperature multiferroicity.

Effect of Quenching Tempering-Post Weld Heat Treatment on the Microstructure and Mechanical Properties of Laser-Arc Hybrid-Welded Boron Steel.

In the present study, we have investigated the effect of post-welding heat treatment (PWHT) of quenching and tempering (QT) on the microstructure and mechanical properties of welded boron steel joints processed using laser-arc hybrid welding on two commercial filler materials, SM80 (Type-I) and ZH120 (Type-II). The microstructure and mechanical properties of the weld joints were characterized via optical microscopy, Vickers microhardness, and the uniaxial tensile test. The macrostructure of the weld joint was composed of a fusion zone (FZ), heat-affected zone (HAZ), and base metal zone (BMZ). After the QT-PWHT, the QT specimens revealed the V-shape hardness distribution across the weld joint, while the as-welded specimen exhibited the M-shape hardness distribution. As a result, the QT specimens revealed the premature fracture with little reduction in the area at the interface between the HAZ and FZ, while the as-welded specimen exhibited the local necking and rupture in the BMZ. In addition, the Type-II filler material with a greater value of equivalent carbon content was rarely influenced by the tempering, maintaining its hardness in the as-quenched status, while the Type-I filler material showed a gradual decrease in hardness with the tempering time. The results demonstrate that the Type-II weld joint outperformed the Type-I weld joint in terms of the structural integrity of welded parts.

Ferroelectric order associated with ordered occupancy at the octahedral site of the inverse spinel structure of multiferroicNiFe2O4

We report a ferroelectric order at ~ 98 K for NiFe2O4, which carries an inverse spinel structure with a centrosymmetric Fd3m structure at room temperature. The value of spontaneous electric polarization is considerably high as ~ 0.29 {\mu}C/cm2 for 5 kV/cm poling field. The electric polarization decreases considerably (~ 17 %) around liquid nitrogen temperature upon application of 50 kOe field, proposing a significant magnetoelectric coupling. The synchrotron diffraction studies confirm a structural transition at ~ 98 K to a noncentrosymmetric structure of P4122 space group. The occurrence of polar order is associated with an ordered occupancy of Ni and Fe atoms at the octahedral sites of the P4122 structure, instead of random occupancies at the octahedral site of the inverse spinel structure. The results propose that NiFe2O4 is a new type-II multiferroic material.

Pressure-induced Lifshitz transition in the type II Dirac semimetal PtTe2

Numerous exotic properties have been discovered in Dirac Semimetals (DSMs) and Weyl Semimetals (WSMs). In a given DSM/WSM, the Dirac/Weyl nodes usually coexist with other bulk states, making their respective contribution elusive. In this work, we distinguish the role of bulk states from the tilted Dirac nodes on the transport properties in DSMs. Specifically, we applied pressure to a type-II DSM material, PtTe2, and studied its pressure modified electronic and lattice structure systematically by using in situ transport measurements and X-ray diffraction (XRD). A pressure-induced transition at about 20 GPa is revealed in the transport properties, while the layered lattice structure is robust against pressure as illustrated in XRD measurement results. Density functional theory (DFT) calculations suggest that this is originated from the Lifshitz transition in the bulk states. Our findings provide evidence to identify the bulk states’ influence on transport from the topologically-protected DSM states in the DSM material.

Spin-orbit-stable type-II nodal line band crossing in n -doped monolayer MoX2 ( X=S , Se,Te)

1H-phase monolayer transition-metal dichalcogenides have spin-split electronic band structures near the K(K') point due to strong spin-orbit coupling and intrinsic inversion symmetry breaking. In case of the monolayer MoX2 (X=S, Se, Te), the spin-split conduction bands cross near the K(K') point. We show that the band crossing occurs not only along the high symmetry directions, but also along arbitrary directions due to a mirror reflection symmetry of the monolayer. As a result, n-doped monolayer MoX2 is a two-dimensional type-II nodal-line material.

Targeting low-cost type-II heterostructures: Synthesis, structure and photoreactivity

One design strategy by which to iterate the photocatalytic efficiency of semiconducting nanomaterials for harvesting solar energy involves the synthesis of type-II heterostructured materials. In this article, a straightforward, facile and environmentally benign route to heterostructures in which SnO2 nanospheres are capped by PbS nanocubes is reported. It offers to address current limitations to photocatalytic efficiency brought about by electron-hole recombination and narrow photoresponsive ranges in many existing systems. PbS nanocubes are grown in the presence of preformed SnO2 nanospheres by functionalizing the surface of the latter using cetyltrimethylammonium bromide (CTAB). Heterostructure formation is confirmed by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area analysis, and transmission electron microscopy (TEM) analysis. Rietveld refinement has been exploited to simultaneously elucidate the atomic and microstructures of these materials, allowing the quantitative determination of particle structure and stability. The combination of narrow band-gap semiconductor (PbS) and wide band-gap semiconductor (SnO2) endows the heterostructured nanomaterial with potential as a photocatalyst and, in the degradation of Rhodamine B (RhB) dye under solar simulation, it showed superior photocatalytic activity to that of its separate SnO2 and PbS components. A strong type-II interaction is demonstrated by the heterostructure and a charge separation mechanism has been utilized to clarify this behaviour.